At this moment I don't own a forum yet but I am already a moderator of one forum but I am not doing that much yet because there are no fights nor violations happening on the site. You may say that sounds boring but the forum is doing well even though everyone were everyone were happy with each other. I believe there are only 2 moderators on that site but we managed to be civil with each other. If I will have my own forum, the number of staff I will need depends n the number of the active members. I will hire more if I will have an overwhelming number of members.
Right now I don't have a forum either, but a social networking site that is set much alike a forum website, hence needing moderation.
As this is is a new project, I'm simply applying the same strategies used when I ran forums in the past; one admin (myself) a super-moderator with admin rights, and a moderator.
Most startup forum work fine with 2-3 staff members for as long as the forum remains under 100 members. Beyond this milestone, or when the forum is pretty busy before reaching this, then adding more moderators, perhaps one per category, is a good way to keep everything in control until the need of more staff comes up.
When we have big events like a blown up contest intended for the website, we hire temps to help us handle the project. For ordinary times, we only have 1 mod on a permanent basis and some 1 or 2 additional mods on part time. It's a not easy to ask for a budget on that particular purpose because we had to submit a justification. There are times that I myself would be handling the moderator job when no one is around to attend to our website. And I get easily annoyed with that.